feat(feed/chat): lazy-render + pagination for long scrolls

Server pagination
  - blockchain.PostsByAuthor signature extended with beforeTs int64;
    passing 0 keeps the previous "everything, newest first" behaviour,
    non-zero skips posts with CreatedAt >= beforeTs so clients can
    paginate older results.
  - node.FeedConfig.PostsByAuthor callback type updated; the two
    /feed endpoints that use it (timeline + author) now accept
    `?before=<unix_seconds>` and forward it through. /feed/author
    limit default dropped from 50 to 30 to match the client's page
    size.
  - node/api_common.go: new queryInt64 helper for parsing the cursor
    param safely (matches the queryInt pattern already used).

Client infinite scroll (Feed tab)
  - lib/feed.ts: fetchTimeline / fetchAuthorPosts accept
    `{limit?, before?}` options. Old signatures still work for other
    callers (fetchForYou / fetchTrending / fetchHashtag) — those are
    ranked feeds that don't have a stable cursor so they stay
    single-shot.
  - feed/index.tsx: tracks loadingMore / exhausted state. onEndReached
    (threshold 0.6) fires loadMore() which fetches the next 20 posts
    using the oldest currently-loaded post's created_at as `before`.
    Deduplicates on post_id before appending. Stops when the server
    returns < PAGE_SIZE items. ListFooterComponent shows a small
    spinner during paginated fetches.
  - FlatList lazy-render tuning on all feed lists (index + hashtag):
    initialNumToRender:10, maxToRenderPerBatch:8, windowSize:7,
    removeClippedSubviews — first paint stays quick even with 100+
    posts loaded.

Chat lazy render
  - chats/[id].tsx FlatList: initialNumToRender:25 (~1.5 screens),
    maxToRenderPerBatch:12, windowSize:10, removeClippedSubviews.
    Keeps initial chat open snappy on conversations with thousands
    of messages; RN re-renders a small window around the viewport
    and drops the rest.

Tests
  - chain_test.go updated for new PostsByAuthor signature.
  - All 7 Go packages green.
  - tsc --noEmit clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client-app/app/(app)/feed/index.tsx

@@ -46,8 +46,12 @@ export default function FeedScreen() {
   const [likedSet, setLikedSet] = useState<Set<string>>(new Set());
   const [loading, setLoading] = useState(false);
   const [refreshing, setRefreshing] = useState(false);
+  const [loadingMore, setLoadingMore] = useState(false);
+  const [exhausted, setExhausted]     = useState(false);
   const [error, setError]     = useState<string | null>(null);
 
+  const PAGE_SIZE = 20;
+
   // Guard against rapid tab switches overwriting each other's results.
   const requestRef = useRef(0);
 
@@ -56,19 +60,20 @@ export default function FeedScreen() {
     if (isRefresh) setRefreshing(true);
     else setLoading(true);
     setError(null);
+    setExhausted(false);
 
     const seq = ++requestRef.current;
     try {
       let items: FeedPostItem[] = [];
       switch (tab) {
         case 'following':
-          items = await fetchTimeline(keyFile.pub_key, 40);
+          items = await fetchTimeline(keyFile.pub_key, { limit: PAGE_SIZE });
           break;
         case 'foryou':
-          items = await fetchForYou(keyFile.pub_key, 40);
+          items = await fetchForYou(keyFile.pub_key, PAGE_SIZE);
           break;
         case 'trending':
-          items = await fetchTrending(24, 40);
+          items = await fetchTrending(24, PAGE_SIZE);
           break;
       }
       if (seq !== requestRef.current) return;   // stale response
@@ -80,6 +85,9 @@ export default function FeedScreen() {
       }
 
       setPosts(items);
+      // If the server returned fewer than PAGE_SIZE, we already have
+      // everything — disable further paginated fetches.
+      if (items.length < PAGE_SIZE) setExhausted(true);
 
       // Batch-fetch liked_by_me (bounded concurrency — 6 at a time).
       const liked = new Set<string>();
@@ -102,6 +110,7 @@ export default function FeedScreen() {
       // testable; in production this path shows the empty state.
       if (/Network request failed|→\s*404/.test(msg)) {
         setPosts(getDevSeedFeed());
+        setExhausted(true);
       } else {
         setError(msg);
       }
@@ -112,6 +121,57 @@ export default function FeedScreen() {
     }
   }, [keyFile, tab]);
 
+  /**
+   * loadMore — paginate older posts when the user scrolls to the end
+   * of the list. Only the "following" and "foryou"/trending-less-useful
+   * paths actually support server-side pagination via the `before`
+   * cursor; foryou/trending return their ranked top-N which is by
+   * design not paginated (users very rarely scroll past 20 hot posts).
+   *
+   * We key the next page off the oldest post currently in state. If
+   * the server returns less than PAGE_SIZE items, we mark the list as
+   * exhausted to stop further fetches.
+   */
+  const loadMore = useCallback(async () => {
+    if (!keyFile || loadingMore || exhausted || refreshing || loading) return;
+    if (posts.length === 0) return;
+    // foryou / trending are ranked, not ordered — no stable cursor to
+    // paginate against in v2.0.0. Skip.
+    if (tab === 'foryou' || tab === 'trending') return;
+
+    const oldest = posts[posts.length - 1];
+    const before = oldest?.created_at;
+    if (!before) return;
+
+    setLoadingMore(true);
+    const seq = requestRef.current; // don't bump — this is additive
+    try {
+      const next = await fetchTimeline(keyFile.pub_key, {
+        limit: PAGE_SIZE, before,
+      });
+      if (seq !== requestRef.current) return;
+      if (next.length === 0) {
+        setExhausted(true);
+        return;
+      }
+      // Dedup by post_id (could overlap on the boundary ts).
+      setPosts(prev => {
+        const have = new Set(prev.map(p => p.post_id));
+        const merged = [...prev];
+        for (const p of next) {
+          if (!have.has(p.post_id)) merged.push(p);
+        }
+        return merged;
+      });
+      if (next.length < PAGE_SIZE) setExhausted(true);
+    } catch {
+      // Don't escalate to error UI for pagination failures — just stop.
+      setExhausted(true);
+    } finally {
+      setLoadingMore(false);
+    }
+  }, [keyFile, loadingMore, exhausted, refreshing, loading, posts, tab]);
+
   useEffect(() => { loadPosts(false); }, [loadPosts]);
 
   const onStatsChanged = useCallback(async (postID: string) => {
@@ -219,6 +279,22 @@ export default function FeedScreen() {
           />
         )}
         ItemSeparatorComponent={PostSeparator}
+        onEndReached={loadMore}
+        onEndReachedThreshold={0.6}
+        ListFooterComponent={
+          loadingMore ? (
+            <View style={{ paddingVertical: 20, alignItems: 'center' }}>
+              <ActivityIndicator color="#1d9bf0" size="small" />
+            </View>
+          ) : null
+        }
+        // Lazy-render tuning: start with one viewport's worth of posts,
+        // keep a small window around the visible area. Works together
+        // with onEndReached pagination for smooth long-feed scroll.
+        initialNumToRender={10}
+        maxToRenderPerBatch={8}
+        windowSize={7}
+        removeClippedSubviews
         refreshControl={
           <RefreshControl
             refreshing={refreshing}

client-app/app/(app)/feed/tag/[tag].tsx

@@ -95,6 +95,10 @@ export default function HashtagScreen() {
           />
         )}
         ItemSeparatorComponent={PostSeparator}
+        initialNumToRender={10}
+        maxToRenderPerBatch={8}
+        windowSize={7}
+        removeClippedSubviews
         refreshControl={
           <RefreshControl
             refreshing={refreshing}